1. Field of the Invention
The present invention relates to a highly reliable rechargeable lithium battery using intercalation and deintercalation reactions of lithium ion in charging and discharging.
The rechargeable lithium battery using intercalation and deintercalation reactions of lithium ion in charging and discharging will be hereinafter simply referred to as rechargeable lithium battery. And the rechargeable lithium battery in the present invention is meant to include a lithium ion battery.
More particularly, the present invention relates to an improved, highly reliable rechargeable lithium battery provided with a specific electrolyte, which stably and continuously exhibits desirable battery performance without being deteriorated even upon the repetition of the charging and discharging cycle over a long period of time and has a prolonged charging and discharging cycle life.
2. Related Background Art
In recent years, global warming from the so-called greenhouse effect has been predicted due to increased level of atmospheric CO.sub.2. To prevent this warming phenomenon from further developing, there is a tendency to restrict the construction of new steam-power generation plants which exhaust a large quantity of CO.sub.2.
Under these circumstances, proposals have been made to institute load leveling in order to effectively utilize power. Load leveling involves the installation of rechargeable batteries at general locations to serve a storage for surplus power unused in the night, known as dump power. The power thus stored is available in the day time when the power demand is increased, leveling the load requirements in terms of power generation.
Separately, there is an increased societal demand for developing a high performance rechargeable battery with a high energy density for an electric vehicle which would not exhaust air polluting substances. There is further increased societal demand for developing a miniature, lightweight, high performance rechargeable battery usable as a power source for portable instruments such as small personal computers, word processors, video cameras, and pocket telephones.
In order to attain such a miniature and light weight rechargeable battery, various studies have been made of a rechargeable lithium battery which would allow the application of a high voltage and which would excel in energy density. For instance, use of a lithium-graphite intercalation compound as an anode active material in a rechargeable battery has been proposed (see, Journal of the Electrochemical Society, 117, 222, (1970)).
Since then, public attention has focused on a rocking chair type lithium ion battery. And various studies have been made in order to develop such a rocking chair type lithium ion battery. The rocking chair type lithium ion battery is typically configured such that a carbonous material is used as an anode active material and an intercalation compound intercalated with lithium ion is used as a cathode active material. The and lithium ion is intercalated into the six-membered network layer planes provided by carbon atoms to store in the battery reaction upon charging. Presently, several rocking chair type lithium ion batteries are known having such a configuration, which are practically usable. In these lithium ion rechargeable batteries, the carbonous material serving as a host for allowing lithium ion as a guest to insert or release is used as the anode active material to prevent the growth of a lithium dendrite so that the charging and discharging cycle life is prolonged.
However, in any of these lithium ion batteries in which a carbonous material is used as the anode active material to store lithium atom therein, the discharge capacity capable of being stably provided upon the repetition of the charging and discharging cycle is not beyond the theoretical electric capacity of the graphite intercalation compound to store one lithium atom in six carbon atoms, and therefore, there can be attained a mere electric capacity capable of storing one lithium atom in 10 carbon atoms in a practical range in terms of the repetition number of the charging and discharging cycle.
In this respect, based on the constitution of the foregoing lithium ion battery in which a carbonous material is used as the anode active material, although it is satisfactory in terms of the charging and discharging cycle life, there cannot be attained a desirable energy density similar to that in a primary lithium battery in which a lithium metal itself is used as the anode active material.
As for anode with an anode active material comprising a carbonous material capable of storing lithium atom therein, an attempt has been made to make it such that it can store a large amount of lithium atom so as to attain an increased battery capacity. However, this is not realistic because problems result, wherein as the charging and discharging cycle proceeds, an insulating film is formed on the surface of the anode due to chemical reaction with an electrolyte solution to raise the impedance of the anode. In addition, the electrolytic solution is gradually decomposed as the Charging operation is repeated, resulting in shortening the charging and discharging cycle life.
Separately, various studies have been made of a rechargeable lithium battery having a high electric capacity in which a metallic lithium is used as the anode. However, such a rechargeable lithium battery is problematic in that lithium is often deposited in a dendritic state (that is, in the form of a dendrite) on the anode during the charging operation, resulting in causing internal-shorts between the anode and the cathode upon repeating the charging and discharging cycle, wherein there cannot be attained a sufficient charging and discharging cycle life.
Particularly, once a lithium dendrite forms on the anode, the lithium dendrite is liable to gradually grow when the charging operation is repeated, resulting in causing internal-shorts between the anode and the cathode. When the anode is internally shorted with the cathode, the energy possessed by the battery is rapidly consumed at the internally shorted portion resulting in problems such that the battery is heated or the solvent of the electrolyte solution is decomposed by virtue of heat to generate gas, thereby raising the inner pressure of the battery. These problems result in damaging the rechargeable lithium battery or/and shortening the lifetime of the battery.
The experimental studies by the present inventors of the occurrence of such problems as above described provided findings as will be described in the following. That is, lithium deposited upon the charging operation is very active, and because of this, the deposited lithium readily reacts with an electrolyte solution or impurities such as water or an organic solvent contained in the electrolyte solution to form an insulating film on the surface of the anode. The insulating film thus formed on the surface of the anode is not uniform. Because of this, upon charging, the electric field converges at portions on the surface of the anode where lithium is locally deposited in a dendritic state and the lithium dendrite deposit often reaches the cathode to result in causing the internal-shorts between the anode and the cathode. In addition, the electrolyte of the electrolyte solution is often dissociated to produce a dissociated electrolyte serving as a polymerization initiator. This dissociated electrolyte polymerizes the organic solvent contained in the electrolyte solution to cause the formation of a polymerized product, which sometimes results in not only raising the internal impedance of the battery but also decomposing the electrolyte solution. As a result, the charging and discharging cycle life of the rechargeable lithium battery is often shortened.
Hence, based on the constitution of the foregoing rechargeable lithium battery, it is difficult to effectively prevent the generation of a dendrite and it is also difficult to realize a practically usable rechargeable lithium battery which stably and continuously exhibits a high battery performance.
In order to attain a high performance rechargeable lithium battery while having a due care about the foregoing situation, there has been proposed a manner of using a lithium alloy such as lithium-aluminum alloy as the anode for a rechargeable lithium battery. However, this manner is not effective in attaining a high performance rechargeable lithium battery having a long charging and discharging cycle life.
Japanese Unexamined Patent Publications Nos. 13264/1988, 114057/1988, 47381/1993, and 190171/1993 disclose various lithium alloys to be used as the anode for a rechargeable lithium battery. In addition, Japanese Unexamined Patent Publication No. 234585/1993 discloses that the anode for a rechargeable lithium battery is constituted by a lithium metal having a powdery metal, which poorly forms an intermetallic compound with said lithium metal, uniformly deposited on the surface thereof. However, the use of any of the materials as the anode constituent disclosed in these documents is not decisively ensured to attain a desirable anode for a rechargeable lithium battery, having a markedly prolonged lifetime.
The Journal of Applied Electrochemistry, 22, 620-627 (1992) discloses a rechargeable lithium battery in which the anode is constituted by an aluminum foil having a surface applied with etching treatment. However, the rechargeable lithium battery disclosed in this document is problematic in that when the charging and discharging cycle is repeated as many as that practically conducted for the ordinary rechargeable battery, problems are liable to result in that as the charging and discharging cycle is repeated, the aluminum foil is repeatedly expanded and shrunk and cracks, resulting in causing a reduction in the current collecting performance, wherein the growth of a dendrite is liable to occur. Hence, in accordance with the manner disclosed in this document, there cannot be attained a rechargeable lithium battery having a sufficient charging and discharging cycle life which can be accepted at a practical use level.
Accordingly, there is an increased demand for an improved, highly reliable rechargeable lithium battery which has a long charging and discharging cycle life in the practical use range and has a high battery capacity.